The present invention relates to an ink jet printer, and especially relates to an ink jet printer in which plural recording heads jet out ink of the same color to perform an image recording.
As an image recording apparatus which can record an image not only on a usual record medium, such as paper and cloth, by also on a record medium having poor ink absorbability, such as a resin film and a metal, an ink jet printer has been developed so as to jet ink from nozzles provided in the end surface of recording heads and to make the ink reach on a record medium, and an at the present day the technique to use the ink jet printer has been applied to various technical fields.
In many cases, the ink jet printer is used with a line head type recording technique or a serial head type recording technique. In the line head type recording technique, a recording head is fixed at a position above a record medium, and jets ink onto the record medium being conveyed in a predetermined direction so as to conduct recording (printing). In the serial head type recording technique, a recording head jets ink onto a stopped record medium and conducts recording (printing) while the recording head is moved outward and homeward in a main scanning direction.
In such an ink jet printer, for example, in the case of a line head type, at the time of setting a record medium, at the time of a maintenance, or at a time of conducting a processing operation such as covering nozzles of a recording head with a cap in the case where there is a period of free time after the completion of a previous recording operation until the beginning of the following recording operation, if an impact force is applied onto a pipe to supply ink to a recording head by an operation to shift the recording head upward or downward or by a motion to touch the pipe, the pressure of ink in the recording head fluctuates.
Also, in a serial head type ink jet printer, at the time of setting a record medium, in addition, at the time that a recording head is moved outward and homeward at the time of recording images, the pressure of ink in the recording head fluctuates.
In the above ways, if the pressure of ink in the recording head fluctuates, ink in nozzles of the recording head is inhaled into the inside of the recording head, and simultaneously, outer air is also inhaled and mix into ink. As a result, the following trouble may be caused, ink is not jetted out from the nozzles, ink is extruded from nozzles by pressure and adheres on a nozzle surface in the vicinity of the nozzles, a meniscus of ink cannot be formed on the nozzles or ink cannot be jetted in a normal direction due to an irregular force such as a pulling force caused by a deposit of ink adhering in the vicinity of the nozzles.
In order to prevent the above trouble from occurring, there is a case, for example, that a damper is provided between a recording head and an ink tank or an intermediate tank, and the damper is adapted to absorb pressure fluctuation when the inside pressure of a recording head increases or decreases. Further, in the ink jet printer disclosed in Japanese Patent Unexamined Publication No. 2003-94681, a structure is proposed to provide an air removing device including a exhaust pipe and an exhaust valve at a location of a conventional damper in order to removing collected air.
However, in the case that two or more recording heads are provided in an ink jet printer, if such an air removing device disclosed by the above Japanese patent unexamined publication is provided for each of the two or more recording heads, the size of the ink jet printer itself becomes larger, and the manufacturing cost also increases at the same time. On the other hand, there is also a problem that air collected in a damper is not easily removed unless a device like the air removing device is provided.
Moreover, even if pressure fluctuation of a recording head is absorbed with a damper, the back pressure of each of plural recording heads does not necessarily become the same pressure. Since the amount of ink jetted out from each recording head does not necessarily become equal, a recorded image has a problem that color unevenness is caused.